The 31 series is a diesel (18 wheeler) battery: it is bigger, heavier, more cranking amps, and has more amp hours. I've used them before in my boats. Only problem with the high cranking amps is that any small electrical short that never was a noticeable problem in the past can be turned into a big problem.

I wouldn't necessarily describe them as 18 wheeler batteries. Yes group 31 batteries of any manufacturer are the standard on most Class 8 trucks but a group 31 can be used on anything that it fits in. If you have the room having bigger batteries is a great upgrade to your system. Normally I would say there are better options for less money but I would buy those in a heartbeat for that price. That is a crazy good deal if they truly are brand new and fully charged. Keep in mind that the group 31's are a few inches longer and a couple inches taller than the group 34's you currently have. You will probably have to get new battery boxes. To check how old they are see if there is a sticker or possibly a stamp into the top of the case somewhere. I am not sure how the date code works on Optima's but for most batteries I have seen they use a letter to signify the month and then 01-12 to signify the year.

Yeah, I already emailed the seller and asked what the date of manufacture is on them.

That's a bummer about the size, because my two D34M's fit just about perfectly inside the built-in cooler under the observer seat. The reason I used Optimas when I reworked my battery setup was to do away with the battery boxes.

The length will be the problem. I just found the dimensions, and the D31M is 2.8" longer. They might fit, but would be a tight squeeze. Maybe it's time to chop out that stupid tiny uninsulated cooler. Anyone done this on an older VLX?

I have 2 Kinetik KM-27's that I am putting in to power the stereo and was looking for another AGM battery to put for the starting battery. ALthough they are isolated, they are combined when the alternator is charging both, so I planned on avoiding a regular lead wet cell battery. I wonder if there would be any problem placing one of these for the starting battery and the Kinetiks for the stereo.